Chucks have been familiar for decades and are used, amongst other purposes, for holding sleeves, i.e. rotationally symmetrical workpieces, in a fixed location on machine tools so that an internal thread can be worked into them, the sleeves being subsequently used as connection and sealing elements in gas or oil pipelines. The internal threads to be manufactured are self-sealing, which means that the production of such threads requires a high level of precision.
The larger the external diameter that is to be selected for such sleeves, the larger the internal diameter with which a chuck body of the chuck must be configured in order to be able to accommodate the workpiece that is to be machined. Furthermore, it is necessary for the internal thread to be worked into the inner wall at both open ends of the sleeve, meaning that the sleeve must be swiveled through 180° following the machining of the first internal thread.
It has also been disclosed, for example in the publications DE 1 214 974 B or GB 2,015,391 A, that two or more clamping jaws are to be provided for clamping the sleeve, by means of which the sleeve is positioned centrally in relation to the longitudinal axis of the chuck once the clamping jaws have been advanced. The precision of machining largely depends on the centring of the sleeve in relation to its longitudinal axis, which is aligned flush with the longitudinal axis of the machine tool.
In what are referred to as two-jaw indexing chucks, it has proved to be a disadvantage that the positioning of the workpiece to be machined is extremely difficult to accomplish, because the lower clamping jaw in the direction of the force of gravity is fixedly supported in the chuck body or on a pivoting ring articulated in a rotating arrangement on the chuck body, with the effect that it is impossible to regulate the height of this clamping jaw. Although the opposite clamping jaw can move radially in relation to the chuck body, it is not possible to lift the sleeve in the event that the centre of the sleeve is arranged below the longitudinal axis of the chuck body, meaning that the lower chuck jaw must subsequently be shimmed using discs, spacer plates and the like, the thickness of which must be adapted to the amount of deviation. Such adjustment measures are time-consuming and have to be repeated each time a new batch of sleeves is to be machined.
Although two-jaw indexing chucks are capable of accommodating sleeves with large internal diameters, and the sleeves can also be rotated through 180° within the chuck body, such two-jaw indexing chucks do suffer from the disadvantage that the machining forces that occur, which are in particular vectored perpendicular to the chucking direction, cannot be optimally supported, which means that the machining forces tend to push the sleeve out of the central position. Furthermore, the workpiece is compressed between two clamping jaws. The greater the chucking forces, the greater the risk that the workpiece could undergo elastic or even plastic deformation. This leads to machining errors or an unusable sleeve.
However, as soon as additional clamping jaws are provided in the area of the lateral or horizontal planes, such large sized sleeves can no longer be rotated, because these clamping jaws take up the space required for turning the sleeve.
If, consequently, several clamping jaws, for example three clamping jaws, are used, it is only possible to machine or clamp sleeves with smaller dimensions.
DE 1 214 974 discloses a two-jaw or multi-jaw chuck by means of which a wall thickness of the chuck body should be provided that is as even as possible. In particular, the radial advance of the clamping jaws, which must take place synchronously to achieve central clamping of the sleeve, is undertaken with the help of a hydraulic circuit by means of which a clamping piston allocated to each clamping jaw is moved axially in the pivoting ring, with the clamping piston making contact with one of the clamping jaws in such a way as to provide an active driving effect.
U.S. Pat. No. 4,811,963 discloses the design measures that allow the pivoting ring to be rotated through 180° in relation to the chuck body with a clamped sleeve. For this purpose, two axially movable racks are provided in the chuck body. The two racks run parallel to one another in one plane and act jointly on a drive gear that is in an active driving connection with one of the pivot pins of the pivoting ring. The two racks are moved in opposite directions to one another, and each stroke of the particular rack causes the drive gear to rotate, as a result of which the pivot pin is rotated.
As a result, the pivoting ring can exclusively be moved through 180°, because subsequently to that the racks are in their end position and must be pulled back to the initial position, with the effect that the pivoting ring is permanently rotated through 180°.
The already disclosed multi-jaw indexing chucks suffer from the disadvantage that their external diameter is at least 750 mm and internal diameter about 185 mm, so as to provide the necessary space for accommodating the racks; because chucks of this kind are to be attached to machine tools with a limited working space, these chucks of prior art take up a considerable amount of space that is no longer available for the working area.
In particular, if three clamping jaws and three compensating jaws ere to be provided so as to reduce the advance and chucking forces of three clamping jaws in such a way that the chucking forces acting on the surface of the workpiece do not deform or cause irreparable damage to the workpiece, but are instead arranged evenly, the internal diameter of the chuck is significantly reduced, with the effect that only workpieces with a small external diameter can be clamped.
Furthermore, the wall thickness of the chuck body in the area of the racks must be made large so as to provide sufficient space to accommodate the racks and corresponding advance elements. It is disadvantageous that chuck bodies of this kind are very large, with an external diameter of 760 mm and internal diameter of 160 mm, for example.